Simply put, you can't become aware that you are diseased by yourself.
They will know if they are feeling weaker, dizzy, confused, overwhelmed, annoyed than their usual selves, or if red dots start showing up all over their skin, or if their skin starts to peel off or fall apart (ew).
But even a Heal check cannot tell exactly what disease a character has, or even if he has a disease. The symptoms could really be anything from poison, to diseases, to curses, to magical effects (nightmares), to your GM being mean to you for not bringing him a beer.
As a GM, i simply try to roleplay the effects, describing how they feel. This one time we had a Magus character wearing a cursed bandana that caused -4 INT, and made him think he was the smartest guy alive, we all simply roleplayed as if he had +4 INT (the "disguised magic effect") and he acted as if that was true.
He, of course, didn't know it was cursed. I had a ton of work to manage his abilities and if his INT was actually 4 points lower instead of 4 points higher. And he only figured something was going on when spells that he had memorized using slots from that higher INT score started to fail. But that took him about 6 sessions to ask someone to cast identify on the bandana and find out about the curse.
Knowledge in a specific area that is related to treating diseases, such as Knowledge(Religion) to know that ghouls carry a disease, or Knowledge(Nature) to know that rats carry diseases, will allow you to possibly know what kind of disease might be affecting a character. But the exact DC and what kind of information is given is up to the GM, and will suffer table variations.
Like Knowledge, Profession, as long as it is related to the cause of the disease (herbalist? doctor?), could give you some information about what is going on. But again, the information and DC is entirely up to your GM and might not work at all.
The spell Diagnose Disease was written to solve your problems.
You determine whether a creature, object, or area carries any sort of disease or infestation (including molds, slimes, and similar hazards), or any exceptional or supernatural effects causing the sickened or nauseated effects. If there is disease present, you know what disease it is and its effects.